In LTE (Long Term Evolution), two radio frame structures are supported; type 1 applicable to FDD (Frequency Divisional Duplexing), and type 2 applicable to TDD. A type 2 frame structure is illustrated in FIG. 5. In both frame structure types, each radio frame of 10 ms is divided into two half-frames of 5 ms, and each half-frame consists of five subframes of length 1 ms. In frame structure type 2, each subframe is either a downlink subframe, an uplink subframe or a special subframe giving rise to different TDD configurations, as shown e.g. in FIG. 6.
The supported uplink-downlink configurations in LTE TDD are listed in Table 1 where, for each subframe in a radio frame, “D” denotes the subframe is reserved for downlink transmissions, “U” denotes the subframe is reserved for uplink transmissions and “S” denotes a special subframe with the three fields DwPTS, GP and UpPTS, which may be seen in FIG. 5. The length of DwPTS and UpPTS is given by Table 1 subject to the total length of DwPTS, GP and UpPTS being equal to 1 ms. Each subframe consists of two slots, each of length 0.5 ms.
Uplink-downlink configurations with both 5 ms and 10 ms downlink-to-uplink switch-point periodicity are supported. In case of 5 ms downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames.
In case of 10 ms downlink-to-uplink switch-point periodicity, the special subframe exists in the first half-frame only. Subframes 0 and 5 and DwPTS are always reserved for downlink transmission. UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission.
In a TDD cell, a TDD configuration is characterized by both uplink-downlink configuration and special subframe configuration. Therefore, the term TDD configuration used hereinafter refers to a combination of uplink-downlink configuration. It should be noted that more TDD configurations than the ones listed in table 1 may be introduced in the future. The herein suggested solution is not limited to the existing TDD configurations, rather it is equally applicable to new configurations defined in future.
TABLE 1Uplink-downlink configurationsDownlink-to-Uplink Switch-Subframe numberpoint periodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
A dynamic UL DL sub-frame configuration is being studied in 3GPP, and the Ministry of Industry and Information Technology (MIIT) of China shows great interest. In the latter, dynamic uplink downlink sub-frame configuration, referred to as “dynamic TDD” for simplicity later on, is identified as one of the key features to improve the performance in hot spot and indoor scenario for LTE TDD systems. The dynamic UL DL sub-frame configuration is also interchangeably denoted “flexible UL DL sub-frame configuration” or “dynamic TDD configuration” or “flexible TDD configuration”.
With dynamic TDD, one cell can dynamically change the direction of the flexible subframes. Hereinafter, a flexible subframe configured or scheduled for downlink or uplink data transmission is referred to as a flexible downlink subframe or a flexible uplink subframe respectively, whereas a subframe that is fixed for downlink or uplink data transmission is referred to as a normal downlink subframe or normal uplink subframe. The normal downlink subframes and normal uplink subframes may also be referred to as non-flexible subframes hereinafter. As a cell can dynamically change the transmit direction of a flexible subframe, the two cells that are neighbors to each other can be configured with different uplink downlink subframe configurations. FIG. 6 exemplifies the UL to DL interference, i.e., UE to UE interference, when the two neighboring cells are configured with different UL DL subframe configurations. Cells A and B are configured with TDD subframe configuration 1 and 2 respectively. In Cell B, the normal subframes Subframe 0, 1, 5 and 6 experience interference from the downlink transmission of the corresponding downlink subframes in Cell A, while the flexible downlink subframes 3, 4, 8 and 9 experience interference from the uplink transmission of UE 1 in Cell A. The experienced interference can be quite different between the mentioned normal and flexible downlink subframes.
CSI (Channel State Information) measurements include measurements of CQI (Channel Quality Indicator), PMI (Pre-coding Matrix Indicator) and RI (Rank Indicator). In LTE Rel-8/9, the CSI measurements are based on CRSs (Cell specific Reference Signals) which are transmitted in each downlink subframe across the whole frequency band. In Rel-10/11, new reference signals are introduced for CSI measurement, i.e. CSI-RS (Channel State Information-Reference Signal). In LTE-Rel-12, a non-backward compatible NCT (New Carrier Type) will be introduced. For NCT, the density of CRSs will be reduced and will only used for frequency and time tracking and will not be used for data demodulation or CSI measurement. The downlink CQI, PMI and rank could be based on the existing CSI-RS. However, there are also possibilities to introduce other kinds of reference signals for the purpose of CSI measurement. This herein suggested solution is applicable for both cases, which is one of its advantages.
In presence of dynamic TDD, the normal downlink subframes and the flexible downlink subframes experience different interferences. In the normal downlink subframes, a scheduled UE experiences the traditional DL to DL interference. In the flexible downlink subframes, a scheduled UE may experience either or both of the DL to DL interference and the UL to DL interference depending on whether the neighboring cells schedule the same/corresponding subframe for downlink or uplink data transmission. For a certain user scheduled in a flexible downlink subframe, the experienced interference situation may be quite different from that of a user scheduled in a normal downlink subframe. Using the universal CSI measurement for both normal and flexible downlink subframes may result in link adaptation performance degradation for the UE in general.
Further, generally, for fixed TDD subframes, i e subframes where the direction of transmission is not variable from one radio frame to another, a UE may apply information provided in advance, e g in a measurement request, for locating time-frequency resource elements (TFREs) where reference signals are transmitted, upon which the measurements are to be performed. Such information may, for example, be specification of a measurement bandwidth and/or a measurement pattern. However, for flexible TDD subframes, where the transmission direction may change from one frame to another, such information may not be available to, or useful for the UE, as the UE may not know in advance whether the flexible TDD subframe is an UL or DL subframe. This may, for example, be the case when a measurement occasion of a requested measurement falls in the flexible TDD subframe. Further, the reference signals available for CSI measurements in the flexible TDD subframe may vary depending e.g. on carrier type and/or measurement configuration. For example, a CRS or a CSI-RS may be transmitted in the flexible TDD subframe, or, if the UE is assigned to a carrier of a new carrier type, there may be no reference signals or fewer reference signals transmitted in the flexible TDD subframe. There is thus a need for a mechanism that enables a UE to perform adequate CSI measurements regardless of what is the actual configuration of the flexible TDD subframe.